1. Field of the Invention
The present invention relates to a transaxle for auxiliary drive wheels of a multi-wheel drive vehicle having main drive wheels and auxiliary drive wheels, wherein an auxiliary transaxle for driving the auxiliary drive wheels is automatically selectively put into either a drive-on mode for driving the auxiliary drive wheels by power transmitted from a main transaxle for main drive wheels or a drive-off mode for isolating the auxiliary drive wheels from the power transmitted from the transaxle for main drive wheels.
2. Related Art
Conventionally, as disclosed in Japanese Laid Open Gazette No. 2002-59754, there is a well-known six-wheel drive utility vehicle as an example of multi-wheel drive vehicles. The six drive wheels of the vehicle are left and right rear main drive wheels, left and right middle main drive wheels, and left and right front auxiliary drive wheels. The vehicle is equipped with a first transaxle for the rear main drive wheels, a second transaxle for the middle main drive wheels, and an auxiliary transaxle for the front auxiliary drive wheels. The front auxiliary drive wheels are steerable. The vehicle also comprises a transmission such as a CVT (continuously variable transmission) for transmitting power from a prime mover to the first transaxle.
The first transaxle is provided with a PTO casing through which the power of the prime mover is distributed between the second transaxle and the auxiliary transaxle. A PTO shaft extended from the PTO casing is drivingly connected to the auxiliary transaxle via a propeller shaft, universal joints and a clutch. The clutch is optionally manipulated so that the clutch is selectively engaged for driving the auxiliary drive wheels by the power transmitted from the PTO casing, i.e., for driving all of the six wheels, or disengaged for isolating the auxiliary drive wheels from the power transmitted from the PTO casing, i.e., for driving only the four main drive wheels.
The problem of the vehicle is to require an operator's troublesome manipulation. If the manipulation is mistaken or wrong, the vehicle may unexpectedly travel by the six-wheel drive, thereby causing waste energy consumption, or causing heavy steering operation.
Then, as disclosed in Japanese Laid-Open Gazette No. Sho 61-27318, there is a well-known conventional transaxle. If this transaxle serves as the auxiliary transaxle of the above vehicle, an operator does not have to manipulate the clutch. The transaxle has a clutch system, which is normally clutched off for driving a vehicle by the driving force of only main drive wheels, and automatically clutched on for driving auxiliary drive wheels in addition to the main drive wheels only when the main drive wheel slips.
In this regard, the transaxle supporting left and right auxiliary drive wheels has a drive shaft interlocking with main drive wheels of a vehicle, and has a pair of left and right two-way clutches, each of which is interposed between the drive shaft and a shaft interlocking with each of the left and right auxiliary drive wheels. Each of the two-way clutches comprises concentric inner and outer rings. The inner ring receives rotary force from the drive shaft driven synchronously to the main drive wheels, and the outer ring receives rotary force from the auxiliary drive wheel. The two-way clutch further comprises a cage holding rollers, which is interposed between the inner and outer rings. The cage is constantly frictionally resisted so as to delay the rotation of the inner ring after the rotation of the outer ring, whereby the outer ring normally rotates together with the auxiliary drive wheel freely from the rotation force of the inner ring. If the main drive wheel slips so as to reduce the rotary speed of the auxiliary drive wheels, the clutches are engaged so as to transmit the rotary force of the inner rings to the outer rings, thereby driving the auxiliary drive wheels by the rotary force from the main drive wheels side. Whether the vehicle travels forward or backward, the effect is obtained due to the two-way clutches.
However, if load on the auxiliary drive wheel is large during the slip of the main drive wheel, the rollers may be firmly engaged between the inner and outer rings so that they cannot separate the inner and outer rings from each other after the main drive wheel escapes from slipping. If the problem happens, the vehicle has to travel with unexpected drive of the auxiliary drive wheels, thereby reducing driving efficiency and steering capacity.